Liquid crystals have been used in the past in a wide variety of electro-optic and thermo-optic light modulating application. These include, in particular, electro-optic light modulating applications which require compact, energy-efficient, voltage-controlled light, such as watch and calculator displays. The electro-optic devices utilize the dielectric alignment effect in nematic, cholesteric and smectic phases of the liquid crystal, in which, by virtue of dielectric anisotropy, the average molecular long axis of the liquid crystal takes up a given orientation in an applied electric field. Thermo optic devices accomplish the orientation by simple melting to the isotropic state via a temperature change.
The processes conventionally used for incorporating liquid crystals into a practical display form are generally complex and demanding. Display products are normally produced by sandwiching the liquid crystal material between two sheets of glass having electrically conductive coatings and then sealing the entire peripheral edge of the sandwich structure.
Conventional manufacturing makes it difficult to produce light modulating devices of large size, or having unusual shapes. In an attempt to expand the size and utility of liquid crystal light modulating devices, methods have been suggested for coating liquid crystal material with various polymers or membranes to simplify their handling and generally allow for large sheet construction of display or light modulating materials.
The art and practice of making liquid display devices which contain the liquid crystal materials within a solidified layer of binder polymer have been described as early as 1972 by Elliot, French Patent No. 2,139,537, Benton, U.S. Pat. No. 3,872,050, and Taylor, U.S. Pat. No. 3,970,579. The concept is elegant in its simplicity, however, it has never achieved commercial success. A number of performance shortcomings which limit the utility of these materials have included: Low contrast, short lifetimes, high voltage requirements and low multiplexibility.
More recently many variations and improvements related to the basic technology have been disclosed. U.S. Pat. No. 4,435,047, for example, describes water emulsion methods both for encapsulating nematic liquid crystal material and for making a liquid crystal device using such encapsulated liquid crystal materials. However, there are a number of inherent difficulties one encounters when working with water emulsion or encapsulation systems. These include difficulty in obtaining and holding a uniform droplet size in the emulsion, or microcapsule, poor spreading on plastic, and inability to dissolve and carry important additives in the system such as dyes, plasticizers, or electrical property modifiers.
Another simplified approach was disclosed by Doane et al, U.S. Pat. No. 4,688,900. In this approach, microdroplets of a liquid crystal material were spontaneously formed in a solid epoxy polymer at the time of its polymerization. The cured polymer matrix containing these microdroplets was sandwiched between two layers of glass containing a conductive coating. This approach has simplified the manufacture of light modulating materials over processes using free liquid crystals or encapsulated liquid crystals. The conventional curing needs of polymers such as an epoxy still cause difficulties in the coating and laminating. The materials are very low in viscosity and uncured materials tend to leak from the display sandwich. Lamination in sheet form is possible using spacers, but a stationary oven cure is required. Certain epoxy materials also suffer from limited ultra-violet durability.
Light modulating materials containing micro-droplets of liquid crystal material within a thermo-plastic matrix have also been proposed. While these materials offer ease of fabrication they suffer a number of drawbacks in commercial applications including limited temperature range, fatigue, slow switching times, and limited durability, U.S. Pat. No. 4,671,618. Subsequent polycyanurate cross-linking of the thermoplastic materials as disclosed in, U.S. Pat. No. 4,888,126, has been shown to improve some of the durability shortcomings of the thermoplastic acrylic compounds. Other issues related to commercial viability of these products such as: contrast ratios, multiplexibility, limited temperature range, and high voltage needs, still require improvement before this technology finds widespread application.